Cochlear implants have improved considerably over the course of the past 20 years, however, there is still a great deal of variability on all measures of post-implant performance. The general goal of the experiments proposed is to identify new ways to use electrophysiological measures to better understand the sources of that variability. Previous research from our laboratory has focused on measuring the response of the auditory nerve to electrical stimulation. The proposed studies will expand that early work to include not only measures of neural activity recorded from the auditory nerve but also cortically evoked responses recorded in the same individuals. These responses will then be compared with performance on a broad range of listening tasks including speech and music perception. While much of the research proposed will be conducted with adult implant users, the results will be directly applicable to pediatric populations. Peripheral measures such as the electrically evoked compound action potential (ECAP) recorded using telemetry systems incorporated into commercial cochlear implants have become important tools for both initial programming and monitoring of young children. It is not only possible to record the cortical responses we propose in young children but specific features of these responses have already been shown to follow developmental trends in children and to be indicative of performance (Sharma et al. 2002a;Ponton et al., 2000). The studies outlined in this proposal are important because they expand on previously published work to include assessment of cortical potentials evoked using a change in the pattern of ongoing electrical stimulation, a feature that simulates more closely the type of stimulation that is typical of everyday listening. Many of the proposed experiments focus on patients who use standard cochlear implants, however, experiments are also included that use these techniques in new populations of cochlear implant users such as those patients who use the Hybrid cochlear implant system and patients who receive bilateral cochlear implants. For each subject group, we have proposed experiments addressing the relationship between electrophysiological measures and performance. We will also assess changes that occur over time with continued implant use in both pediatric as well as geriatric populations. We expect that, based on results of the studies proposed, we will be able to more fully characterize the differences we observe across individuals and use that information to assist with clinical decision making, inform candidate selection and influence the ways the speech processor is programmed in order to maximize performance for an individual user.